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The club has moved from perennial cellar dwellers to a competitive national football premiership outfit.
It leapt to a fifth-place finish last year and this year is in position to back that up.
But the improvements run deeper than that.
O'Reilly had been lured to Dunedin because he saw potential for success. However, there was groundwork that needed done before that became evident.
Both on and off the pitch he and the club set about making its operation as professional as possible within the resources it had available.
"It was a big mind shift for everyone," he said.
"That's continuing. We're not there yet, we're continuing to evolve that.
"We were shifting away from two practices a week and a match in the weekend, to an environment where we've got the guys in five, six, seven times a week."
There was a trade-off to every decision and that increased workload was substantial.
It was something many players could not commit to. Many had jobs, study and families which prevented them giving the time required.
Others simply decided they would rather not.
"It's a monstrous commitment, I'm well aware of that.
"It's pretty much dedicating your life to it for six months, so trying to get the players to buy in to that has been tough."
On top of that, retaining the top local talent was difficult.
Many quality players from Dunedin were now spread around the league. Football, work, study and lifestyle opportunities had contributed to that.
That could make forming a competitive squad challenging.
O'Reilly decided to bring in overseas players - initially five Irishmen - to complement the New Zealand and southern talent.
Since then others had joined and three of the original cohort - Conor O'Keeffe, Danny Ledwith and Stephen Last - remain as both players and development officers.
O'Reilly did not categorise them differently. For him they were all Southern United players who wanted to represent the region, regardless of origin.
It is a move that still sparks debate.
The argument against it is that it blocks the pathway for New Zealanders.
"I've done that because I think it's the right thing to do to make sure we're competitive and we've got a really top-class environment for young players to come in to.
"But I'm well aware that's led to some conflict and parts of the football community don't like the fact I've brought in imports."
O'Reilly was fine with that.
People were entitled to their opinion and he understood the reasons they held them.
However, he had to believe in what he was doing.
That was not always easy.
In his first season the side finished bottom of the league and the import criticism was at its height.
He believed that season laid the groundwork for the past two, although that took time to show.
"I won't lie to you, it was difficult at times.
"There was lots of commentary from the community - at times it was like `get them out of here, send them home'.
"And you have to wear that when you're not getting results and just accept that's what people's opinions are."
Being so far away from home could make that tougher.
Family and friends were the big thing he missed.
Alongside that there were the smaller things, such as a "good pint of Guinness" and the overall football atmosphere of Ireland and Europe.
However, he was delighted to have made the move to New Zealand with his partner. He loved the lifestyle in Dunedin and that was enough to keep him from missing home too much.
Despite that it was three years since he had been home and a holiday over winter could be on the cards.
His daughter was born in New Zealand and with a second child on the way in the coming weeks, he hoped to keep them in tune with their Irish roots.
O'Reilly was unsure what the future held.
The club had big decisions to make at the end of this season - although that will be done at a level above him.
He felt there was still room to grow and if it decides to head in that direction he hoped he could be part of it.
However, he was ambitious as well and has hopes of moving on some day to coach at the highest level he could.
For now, though, O'Reilly's attention is focused on tomorrow. Southern United will kick off against Hawke's Bay United at 11am at Sunnyvale Park in its second-to-last match of the season.